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1998 T100 SR5 2WD
14,850 Posts
Discussion Starter · #1 · (Edited)

Toyota Idle Air Control Valve


The rotary solenoid (RS) type IAC valve is located on the throttle body and intake air bypassing the throttle valve passes through it.

In this way the intake air volume bypassing the throttle valve is regulated, controlling the engine speed.

The ECM operates only the IAC valve to perform idle-up and provide feedback for the target idling speed. The IACV receives its power from the EFI relay and ground through the ECM. There are two styles of rotary solenoid IACVs. The older style uses two driver circuits, one driver for each coil. The newer style uses a single driver circuit, one coil is controlled by the ECM while the other coil is always grounded. They are not interchangeable. An easy way to tell which type of rotary solenoid is to use the wiring schematic. The older style has two wires connected to the ECM while the newer type has one connected to the ECM and the other wire connected to ground.

Rotary IACV Operation.


The valve assembly consists of two electrical coils, a permanent magnet mounted on the valve shaft, and a valve. A fail-safe bimetallic strip is fitted to the end of the shaft to operate the valve in the event of electrical failure in the IACV system. Located at the end of the valve shaft, the cylindrical permanent magnet rotates when its two poles are repelled by the magnetism exerted by coils T1 and T2. Anchored to the midsection of the valve shaft, the valve controls the amount of air passing through the bypass port. The valve, valve shaft, and permanent magnet all rotate together.

Strip As shown, each coil is connected to a transistor, T1 and T2 located in the ECM. When transistor T1 turns on, current flows through that coil. The magnetic field of the coil and the magnetic field of the permanent magnet cause the valve to rotate clockwise. When T2 is turned on, the valve rotates counterclockwise.

The ECM varies the on time (duty ratio) for each coil. The difference in strength between the two magnetic fields determines the position of the valve. The frequency is very high, 250Hz. This high frequency helps the valve maintain the correct position for proper airflow.

Single Driver Rotary IACV Operation.

The difference with this type of IACV is that the ECM sends a duty cycle signal to one coil inside the IACV; the other coil is always on. To change the IA
CV position, ECM changes the duty ratio in the controlled coil.

Bimetallic Spring Operation.

If the electrical connector is disconnected or the valve fails electrically, the shaft will rotate to a position determined by the balancing of the permanent magnet with the iron core of the coils and the bi-metal strip. The cold idle will not be as fast as normal and the warm idle will be higher than normal. Using a bimetallic strip allows the IACV to change airflow rate with the change in temperature. The default rpm is approximately 1000 to 1200 RPM once the engine has reached normal operating temperature.

Rotary IACV Controlled Parameters

Engine Starting.
As the engine is started, the ECM opens the IACV to a preprogrammed position based on coolant temperature and sensed rpm.

Warm-up. Once the engine has started, the ECM controls the fast idle based on coolant temperature.


As the engine approaches normal operating temperature, engine speed is gradually reduced. At this time the ECM is comparing actual idle rpm to the target rpm.

Feedback Control. The ECM utilizes a feedback idle air control strategy (which functions very much like the stepper motor IAC system). That is, when the actual engine speed is lower than the target idling speed, the ECM signals the IACV to open. Conversely, when the actual idle speed is higher than the target idle speed, the ECM signal the IACV to close.

Engine Load/Speed Change Estimate Control.

To prevent major loads from changing engine speed significantly, the ECM monitors signals from the neutral start switch (NSW), the air conditioner switch (A/C), headlights or rear window defogger (ELS), and in models equipped with power steering, an oil pressure switch (PS). By monitoring these inputs, the ECM reestablishes target idle speeds accordingly, and adjusts IACV position.

Check Idle Air Control Valve (Step Motor Type)

Before a change in engine speed can occur, the ECM has moved the IACV to compensate for the change in engine load. This feature helps to maintain a stable idle speed under changing load conditions. These speed specifications can be useful when troubleshooting suspected operational problems in the IAC system or related input sensor circuits.

The Rotary Solenoid IAC system utilizes a learned idle air control strategy.

The ECM memorizes the relationship between engine rpm and duty cycle ratio and periodically updates its memory. Over time, engine wear and other variations tend to change these relationships. Because this system is capable of feedback control, it is also capable of memorizing changes in the relationship of duty ratio and engine rpm. The ECM periodically updates its memory to provide more rapid and accurate response to changes in engine rpm.
NOTE: If the battery is disconnected, the some ECM must relearn target step positions.

Feedback (Closed Loop) Idle Air Control

The ECM has a preprogrammed target idle speed that is maintained by the IACV based on feedback from the NE signal. Feedback idle air control occurs any time the throttle is closed and the engine is at normal operating temperature. The target idle speed is programmed in an ECM look up table and varies depending on inputs from the A/C and NSW signals. Any time actual speed varies by greater than 20 RPM from target idle speed, the ECM will adjust the IAC valve position to bring idle speed back on target. The ECM utilizes a feedback idle air control strategy. That is, when the actual engine speed is lower than the target idling speed, the ECM signals the IACV to open. Conversely, when the actual idle speed is higher than the target idle speed, the ECM signal the IACV to close.

PIDs (Data) by Scan Tools. Scan display: IAC DYTY RATIO;

Measurement Item: Intake Air Control Valve Duty Ratio (Opening ratio rotary solenoid type IAC valve, ISC DUTY CYCLE - Idle Speed Control -ISC- Valve percentage opening);

Normal Condition: Idling: 22.5-43 %.


Connect the positive (+) lead from the battery to terminal +B and negative (-) lead to terminal RSC and check that the valve is closed.

Connect the positive (+) lead from the battery to terminal +B and negative (-) lead to terminal RS0 and check that the valve is open.

If operation is not as specified, replace the IAC valve.

Dirty IAC

IAC after Clearing



(a) Initial conditions:
  • Engine at normal operating temperature
  • Idle speed checked correctly
  • Transmission in neutral position
  • A/C switch OFF
(b) Using SST (or suitable jumper*), connect terminals TE1 and E1 of the DLC1. SST 09843-18020

(c) After engine speed is kept at approx. 1,000 rpm for 5 seconds, check that it returns to idle speed*.
If the engine speed operation is not as specified, check the IAC valve, wiring and ECM.

(d) Remove the SST (or suitable jumper*) from the DLC1. SST 09843-18020

20 INSPECT IAC VALVE RESISTANCE NOTICE: "Cold" and "Hot" in the following sentences express the temperature of the coils themselves. "Cold" is from -10°C (14°F) to 50°C (122°F) and "Hot" is from 50°C (122°F) to 100°C (212°F).

(a) Disconnect the IAC valve connector.

(b) Using an ohmmeter, measure the resistance between terminal +B and other terminals (RSC, RSO).

Resistance: Cold: 17.0 - 24.5 Ohm; Hot: 21.5 - 28.5 Ohm. If resistance is not as specified, replace the IAC valve.

(c) Reconnect the IAC valve connector.

97 T100
146 Posts
Hah! Thanks Bam. Wish I'd taken this bit of advice to heart when mine went bad.
"If operation is not as specified, replace the IAC valve."
Cleaning 'em seems to only get you so much farther down the road before they go bad again. Haven't put my new one in yet ... as soon as it stops raining here.

BTW, there are a few vids of my troubleshooting/testing mine on this post.

1998 T100 SR5 2WD
14,850 Posts
Discussion Starter · #3 ·
I'm thinking that one of the things that can shorten the life of the IACV motor is that it tries to turn the vane but can't and ends up damaging some of the windings. At least that's one of my theories. ;)
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